This application claims priority from the following Australian provisional patent applications, the full contents of which are hereby incorporated by cross-reference.
The present invention relates to an applicator for a fluent substance, and in the preferred form to an apparatus for applying a slurry to a substrate, by spattering.
The invention has been developed primarily for use in relation to the production of fibre reinforced cement (FRC) sheet from cementitious slurry, and will be described primarily with reference to that application. However, it will be appreciated that the invention is not limited to this particular field of use.
The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow the significance of it to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an admission that such art is widely known or forms part of common general knowledge in the field.
Various apparatus for applying liquids to substrates are known. For example, one process makes use of a spray bar whereby a liquid coating, such as a paint or primer, is squirted and atomised through spaced apart nozzles, so as to coat the substrate which typically passes progressively beneath the spray bar on a conveyor. One problem with devices of this type is that the relatively fine nozzles required to achieve the degree of atomisation necessary for uniform coating are readily clogged, particularly in the case of slurries containing a solid component in suspension. This results in inconsistent application, and requires frequent cleaning which is time consuming, costly and disruptive to the production process. Atomisation is also problematic in the case of more viscous liquids and slurries.
Another known device is a curtain coater, which makes use of a sheet or curtain of flexible fabric material which drapes over the moving substrate and applies a coating by means of a direct wiping action. However, curtain coaters are prone to inconsistent application, are not well suited for use with slurries, are limited in terms of the speed at which they can operate effectively in a production environment, and are not well adapted to applying relatively thick coatings.
Another known form of applicator is usually referred to as a flood coater, which essentially operates by forming a pool of liquid on the substrate, and spreading the pool over the surface with air jets. Again, however, there are limitations with this technique in terms of the uniformity of application, the viscosity of the liquid or slurry that can be used, and the thickness of the layer or coating that can be applied.
Generally, therefore, these known forms of apparatus are subject to a variety of limitations including susceptibility to clogging, inconsistent application, limitations in speed, limitations in the width of sheet material that the coating can be applied to, limitations in the consistency of the liquid or slurry that can be applied, or some combination of one or more of these shortcomings. They are also typically adapted to apply relatively thin outer surface coatings, as distinct from intermediate layers of substantial thickness as part of a laminated sheet. These limitations render such prior art devices generally unsuitable for use in the manufacture of sheet materials, and particularly FRC sheets, of substantial size and at relatively high speed.
Another known form of apparatus is a spatter coater, which makes use of a rotating roller incorporating a radial array of flexible filaments or bristles to spatter a coating onto a substrate. Spatter coaters are used, for example, to apply surface coatings to clay or masonry tiles, on a production line. Spatter coaters are able to some extent to overcome some of the deficiencies of the other known forms of coating apparatus, especially in terms of clogging. However, in the context of the production of sheet material, known spatter coaters are also subject to inherent limitations.
In particular, known spatter coaters are not able accurately to stop and restart the application process on an intermittent basis, in order to permit precisely controlled coating or laminating. This is especially so with the types of slurries typically used in the production of FRC sheet, because of the relatively runny consistency required to ensure xe2x80x9cself-levellingxe2x80x9d, and the consequential tendency for excess slurry to drip onto the substrate, even if the slurry supply is shut off or the spattering roller is temporarily stopped.
This precise control over intermittent stopping and starting of the application process is particularly important in a high speed production environment where different batches of sheets, having different layers, thicknesses or properties, may be required to run back to back through the coating apparatus and inaccurate transitional control can result in patchy application, or the coating for one batch running over into the following batch of product.
A further difficulty arises due to the fact that, in the manufacture of FRC sheet or other products using cementitious slurries, it is desirable not to stop the supply of slurry to the apparatus itself, as this can result in the stagnation or accumulation of slurry in the apparatus or in upstream parts of the process. This, in turn, can result in overflows, changes in slurry consistency or concentration, settling or sedimentation, or undesirable variations in other process parameters.
Known spattering apparatus do not provide for the precise interruption of the slurry application process, and also do not allow for interruption without stopping the supply of slurry to the apparatus. They are therefore not effective in enabling an accurately controlled intermittent application process, especially in a high speed production environment for sheet materials.
It is an object of the present invention to overcome or ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative.
Accordingly, the invention provides an apparatus for applying a slurry to a substrate, the apparatus including:
a delivery surface disposed to support a layer of slurry;
spattering means adapted to be positioned closely adjacent the delivery surface and being movable so as to spatter the slurry from the delivery surface onto the substrate; and
regulation means for selectively varying or interrupting the flow of slurry from the delivery surface onto the substrate.
The terms xe2x80x9cspatterxe2x80x9d, xe2x80x9cspatteringxe2x80x9d and the like as used herein, are intended to encompass any application technique whereby the slurry is deposited onto a surface or substrate in droplet, globule, particulate or atomised form, whether produced by brushing, flicking, rotating, spraying, agitating, atomising or other dispersion means, and whether propelled by mechanical, electrostatic, hydrostatic, hydrodynamic, gravitational or other means.
Unless the context clearly requires otherwise, throughout the description and the claims, the words xe2x80x98comprisexe2x80x99, xe2x80x98comprisingxe2x80x99, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of xe2x80x9cincluding, but not limited toxe2x80x9d.
Preferably, the apparatus includes a reservoir to contain slurry upstream of the regulation means. Preferably, the reservoir includes an inlet to direct slurry from a supply source and an outlet associated with the regulation means.
The regulation means preferably include a pair of barrier elements selectively movable to define an intermediate clearance space of variable effective cross-sectional flow area, thereby to permit selective regulation of the flow rate of slurry from the reservoir, between the barrier elements, through the outlet, to the delivery surface. Preferably, the barrier elements are selectively adapted, in a closed configuration, to shut off flow between the reservoir and the delivery surface.
Preferably, one of the barrier elements comprises a first cylindrical roller rotatable about a first axis. The other of the barrier elements is preferably a second cylindrical roller rotatable about a second axis, parallel to the first. The rollers are preferably configured to rotate in opposite directions.
The first roller preferably takes the form of a delivery drum, the outer surface of which constitutes the delivery surface. The second roller preferably takes the form of a metering roller selectively movable toward, and away from, the delivery drum.
The apparatus preferably includes a main frame supporting the delivery drum, and a first sub-frame on which the metering roller is mounted, the first sub-frame being rotatable about a third axis parallel to, and spaced from, the second axis, thereby adjustably to displace the metering roller towards, and away from, the delivery drum while maintaining a parallel orientation between them.
In one preferred embodiment, the apparatus includes first hydraulic or pneumatic actuation means extending between the main frame and the first sub-frame for adjustably moving the metering roller and the delivery drum toward or away from each other.
Preferably, the spattering means include a plurality of resiliently flexible elongate spattering elements in the form of bristles, extending radially outwardly from a cylindrical body rotatable about a fourth parallel axis. The body and spattering elements together preferably form a spattering roller.
The apparatus preferably further includes a second sub-frame on which the spattering roller is mounted, the second sub-frame being rotatable about a fifth axis substantially parallel to, and spaced apart from, the fourth axis. A second actuator preferably extends between the main frame and the second sub-frame to effect independently adjustable displacement of the spattering roller towards, and away from, the delivery drum, so as to permit selective variation or interruption of the spattering process, as part of the regulation means.
Preferably, the apparatus further includes a tank for containing a supply of the slurry and a delivery conduit for delivering the slurry from the tank to the reservoir through the inlet.
In one embodiment, the reservoir is defined by a tank positioned immediately above the delivery and metering rollers. In an alternative embodiment, the reservoir is simply a containment region defined between adjacent rollers, preferably the delivery roller and an abutting idler roller, with the metering roller being positioned above the delivery roller.
In a preferred embodiment, the slurry is a cementitious slurry and more preferably, is formed from a mixture of silica, cement, water and optionally other additives.
The apparatus is particularly suitable for applying aesthetic or functional coating layer to an existing substrate. In one embodiment, the slurry is a self levelling dewaterable cementitious slurry with a solids content of between 50% and around 90%. The slurry preferably includes a dewatering aid in a sufficient quantity to permit dewatering of the slurry, preferably through the substrate with or without vacuum assistance.